This invention provides a method of developing radiation sensitive negative resists in a manner which produces a developed resist free from swelling, profile distortion and line distortion.
The resist polymer is applied to the surface of the semiconductor wafer using one of the standard methods of application such as spin coating. The film of resist polymer covering the semiconductor surface is then exposed to radiation from a source such as electron beam, ion beam, or X-ray in order to create the desired resist pattern or image within the film. In the case of a negative resist, the pattern is in the form of a crosslinked polymer which is less soluable than the surrounding nonexposed polymeric material. The pattern or image is subsequently developed by removal of the more soluble, nonexposed polymeric film via use of a solvent.
When images with narrow lines, especially lines less than one micrometer in width, such as those created by electron beam exposure, are developed in negative resists, there is a pronounced tendency for the lines and edges to undergo a characteristic sinusoidal deformation that can be named "snaking." Other observers have described this line phenomenon as "sinuous lines," or "scriggily lines, or wavy lines." These snaky lines form lateral filaments, in some instances which cause bridging of the adjacent lines; in severe cases, snaky lines may not only bridge but merge.
It has been observed that the developed negative resist is swollen and is thicker (higher) than the thickness (elevation) of the initial resist layer. This increased thickness may introduce undesirable anomalies into the developed resist; a developed resist having substantially the thickness of the initial resist layer is preferred.
The article, "Three Dimensional Behavior of Negative Electron Resists" by R. D. Heindenreich and G. W. Kammlott, Polymer Engineering and Science, June 1977, Vol. 17, No. 6. pp. 377-380, is concerned with the influence of swelling in the exposed (irradiated) resist during solvent developing. The polymer used was poly(glycidyl methacrylate-co-ethyl acrylate) (PGMA-co-EA). The developer was a solution mixture of methyl ethyl ketone and ethanol; two proportions were used: 5 MEK:2 ethanol and 5 MEK:1.5 ethanol. The development was by immersion. When the 5:2 solution was immediately followed by the 5:1.5 solution, the exposed resist developed lines were very scriggily. It was concluded that the poor edge definition of the developed resist is the result of swelling of partially crosslinked polymer during exposure to the liquid developer.
The article, "Sol-Gel Behavior and Image Formation in Poly(glycidyl methacrylate) and Its Copolymers with Ethyl Acrylate," by E. D. Feit, M. E. Wurtz and G. W. Kammlott, Journal of Vacuum Science and Technology, 15(3), May/June 1978, pp. 944-947, is concerned with the interaction of the resist and the liquid developer. Seven samples of Poly(glycidyl methacrylate) (PGMA) and eight samples of P(GMA-co-EA) were tested, using a scanning electron beam radiation source. The developer was a solution mixture of ketone and alcohol (species not specified) in proportions ranging from 3:1 to 9:1. In some instances, a high ratio solution immersion was followed by a lower ratio solution immersion. Owing to the swelling of the resist during development, lines deformed and assumed a sinusoidal appearance. It was concluded that faithful reproduction in the resist of a feature written by the electon beam does not depend on molecular parameters of the polymer alone, but rather on factors such as solvent induced swelling, competitive wetting of the substrate and of the polymer by the solvent, and gel rupture by forced development.
The article, "PGMA as a High Resolution, High Sensitivity Negative Electron Beam Resist", by Yoshio Taniguchi et al, Japanese Journal of Applied Physics, Vol. 18, No. 6, June 1979, pp. 1143-1148, is concerned with comparing resist material and concludes that PGMA is, under appropriate conditions, an excellent resist material. The samples were electron beam exposed; the development was by immersion or spraying with a solution mixture of methyl ethyl ketone and ethanol. The developed samples were rinsed for sixty seconds in methyl isobutyl ketone. The optimum developer was 6:1-10:1 solution mixture, with an immersion time of 180 seconds (40 seconds by spraying). It was observed that some resists showed line deformation, referred to as "rough edges", which was attributed to post-baking temperatures.
The article, "Chloromethylated Polystyrene as a Dry-Etch-Resistant Negative Resist for Submicron Technology," by Saburo Imamura, Journal of Electrochemical Society, Vol. 126, No. 9, September 1979, pp. 1628-1630, is concerned with a new resist material, chloromethylated polystyrene (CMS). As a resist material, CMS was irradiated with X-rays and with deep UV-radiation. The exposed CMS resist was developed by dipping into n-amyl acetate solvent for thirty seconds, and then rinsed in isopropyl alcohol for sixty seconds. No mention is made of resist line or edge deformation.
The article, "Molecular Parameters and Lithographic Performance of Poly(chloromethylstyrene)--A High Performance Negative Electron Resist," by H. S. Choong and F. J. Kahn, Journal of Vacuum Science and Technology, 19(4), Nov/Dec 1981, pp. 1121-1126, is concerned with locating a resist material that is the equivalent of the chloromethylated polystyrene of Imamura (above). It is considered that the alkylation agent of Imamura is carcinogenic. It was found that the poly(chloromethylstyrene) polymer was equal to the Imamura polymer. The tests were carried out by exposing the resist with a scanning electron beam. The exposed resist was developed by dipping into n-pentyl acetate, followed by an isopropyl alcohol rinse (as was done by Imamura). It was observed that developed lines were wavy and showed some bridging, attributed to resist swelling and the proximity of the lines.
In U.S. patent application, Ser. No. 491,636, by Robert G. Brault and Leroy J. Miller, mixtures of a good solvent combined with a poor or nonsolvent are utilized to develop the image in order to reduce distortion of the developed image. After development, the imaged polymer is rinsed with a material which is a nonsolvent for the polymer, but which has affinity for the developer solvent. Use of such rinse reduces the amount of residual developer solvent in the image polymer and thus minimizes the distortion of the developed image from the initial image prior to development.
P. C. Deb and S. R. Palit reported the use of a mixture of two nonsolvents to dissolve a polymer in Makromolekulare Chemie, 166, 1973, pp. 227-234. They found that poly(methylmethacrylate) (PMMA) can be dissolved in a mixture of the two nonsolvents, carbon tetrachloride and methyl alcohol, and suggest mechanisms to explain the phenomenon. However, there is no comment concerning use of a mixture of two nonsolvents to dissolve crosslinked polymers. There is no mention of possible use of such nonsolvent mixtures in lithographic processes.
The present invention provides a further improvement in the wet development of irradiated negative resists, so that a developed image is produced which is substantially undistorted from the initial image prior to development.